The present application is based on Japanese priority application No.2000-367727 filed on Dec. 1, 2000, the entire contents of which are hereby incorporated by reference.
This invention generally relates optical semiconductor devices and more particularly to a semiconductor optical amplifier.
An optical-fiber telecommunication system uses an optical amplifier for amplifying optical signals. In recent optical-fiber telecommunication systems that transmit wavelength-multiplexed optical signals, in which a large number of optical elements are used for synthesizing or dividing the optical signals, there is a need of providing a number of semiconductor optical amplifiers of low electric power consumption for compensating for the optical loss that is caused as a result of use of such a large number of optical elements.
In an optical fiber, an optical signal that is transmitted therethrough generally has a random polarization state. Thus, the semiconductor optical amplifier that is used for amplifying optical signals in such an optical fiber has to be a semiconductor optical amplifier of polarization-independent (polarization-dependence free) type.
FIGS. 1A and 1B show the construction of a typical conventional semiconductor optical amplifier 10.
Referring to FIG. 1A, the semiconductor optical amplifier 10 is formed on an n-type InP substrate 11 and has a layered structure that resembles to the structure of a laser diode. Thus, a first cladding layer 12 of an n-type InP is formed on the substrate 11, and a first optical confinement layer 13 of undoped InGaAsP is formed on the first cladding layer 12. Further, an active layer 14 of undoped InGaAs is formed on the first optical confinement layer 13, and a second optical confinement layer 15 of undoped InGaAsP is formed on the active layer 14. Further, a second cladding layer 16 p-type InP and a contact layer 16A of p-type InGaAs are formed consecutively on the second optical confinement layer 15. Furthermore, a p-type electrode 17 is formed on the contact layer 16A and an n-type electrode 18 is formed to a bottom surface of the substrate 11.
Further, the semiconductor optical amplifier 10 has an input end and an output end respectively covered with anti-reflection films 10A and 10B. Thus, when an incident optical beam is introduced to the input end through the anti-reflection film 10A in the state in which a driving bias is applied across the electrodes 17 and 18, the incident optical beam undergoes optical amplification by stimulated emission as it is guided through the active layer 14 to the output end.
FIG. 1B shows the semiconductor optical amplifier 10 in an end view.
Referring to FIG. 1B, the layered structure formed on the substrate 11 and including the cladding layer 12, the optical confinement layer 13, the active layer 14 and the optical confinement layer 15 is subjected to an etching process, and there is formed a mesa stripe that extends in an axial direction of the optical amplifier 10. At both lateral sides of the mesa stripe, it can be seen that there are formed current confinement layers 11A and 11B of n-type InP and current confinement layers 11C and 11D of p-type InP.
When using such a semiconductor optical amplifier 10 in an optical-fiber telecommunication system, it is necessary that the optical amplification is obtained irrespective of the polarization state of the incident optical beam as noted previously. Further, the semiconductor optical amplifier for use in an optical-fiber telecommunication system is required to have a large dynamic range so as to be able to deal with large power fluctuation of the input optical signal. In order to meet for these requirements, the semiconductor optical amplifier 10 has to be able to provide a large fiber-coupled saturated optical power. It should be noted that the fiber-coupled saturation optical power is a quantity defined for the entire system including the semiconductor optical amplifier, an input optical fiber coupled to the semiconductor optical amplifier, an optical system cooperating with the input optical fiber, an output optical fiber coupled to the semiconductor optical amplifier and an optical system cooperating with the output optical fiber, and is defined, based on the fiber-to-fiber gain, in which the loss of the optical systems is taken into consideration, as the value of the fiber-coupled optical power that causes a drop of 3 dB in the fiber-to-fiber gain.
In the case of designing a polarization-independent optical semiconductor device based on the semiconductor optical amplifier 10, the simplest way would be to use a strain-free bulk crystal for the active layer 14 and set the thickness of the active layer 14 to be identical with the width thereof as shown in FIG. 2A, wherein it should be noted that FIG. 2A is an enlarged view showing a part of the mesa-stripe of FIG. 1.
With the construction of FIG. 2A, it should be noted that polarization-independent operation is guaranteed for the optical amplifier in view of the fact that the optical confinement factor becomes the same in the Te-polarization mode in which the electric field oscillates parallel to the surface of the active layer and in the Tm-polarization mode in which the electric field oscillates vertically to the the active layer (xcex93te=xcex93tm), and in view of the fact that the material gain becomes the same in the Te-polarization mode and in the Tm-polarization mode (gte=gtm). Because of this, the product of the optical confinement factor xcex93 and the material gain g becomes the same in any of the two polarization modes (xcex93texc2x7gte=xcex93tmxc2x7gtm), and this guarantees the above-noted polarization independent operation for the optical amplifier.
In the case the thickness of the active layer 14 is thus formed equally with the width in the semiconductor optical amplifier 10 of FIG. 1, on the other hand, it is necessary to form the active layer 14 to have a width of 0.5 xcexcm or less in order to realize a fundamental-mode optical guiding. However, processing of the active layer to such a small size is difficult, and the production of such an optical amplifier has been difficult.
FIG. 3 shows the relationship between the chip-out saturation power represented in the left vertical axis and the thickness of the active layer 14 obtained by the inventor of the present invention. Further, FIG. 3 shows a tensile strain to be introduced into the active layer 14 for realizing the polarization independent operation for the optical amplifier. In FIG. 3, the optical confinement layers 13 and 15 are assumed to have the thickness of 100 nm in semiconductor optical amplifier 10 of FIG. 1, and the calculation was made by setting the width of the active layer 14 to 1.0 xcexcm. The strain introduced into the active layer 14 will be explained later.
FIG. 3 is referred to.
In the case the thickness of the active layer 14 is decreased, it can be seen from FIG. 3 that the value of the chip-out saturation power of the semiconductor optical amplifier 10 is increased. This effect reflects the situation in which the saturated output Ps of semiconductor optical amplifier the 10, represented as
Ps=(wd/xcex93)*(hxcexd)/(xcfx84gxe2x80x2),xe2x80x83xe2x80x83Eq.(1)
is increased as a result of increase of the mode cross-sectional area (wd/xcex93), which in turn is caused as a result of decrease of thickness d of the active layer 14 and further as a result of increase of the carrier lifetime xcfx84. In Eq.(1), it should be noted that w and d represent the width and thickness of the active layer 14 respectively, xcex93 represents the optical confinement factor, h represents the Planck constant, xcexd represents the optical frequency, xcfx84 represents the carrier lifetime in the active layer 14, and gxe2x80x2 represents the differential gain.
In Eq.(1), it should be noted that the value of the parameter d is decreased in the representation of the mode cross-sectional area wd/xcex93 when the thickness d of the active layer is decreased. However, the optical confinement factor xcex93 decreases more sharply with the decrease of the thickness d, and there occurs, as a whole, an increase in the cross-sectional area wd/xcex93. Thereby, the saturated output Ps is increased. Also, in Eq.(1), carrier lifetime xcfx84 is represented in terms of carrier density N in the active layer 14, non-optical recombination coefficient A, optical recombination coefficient B and Auger recombination coefficient C as
1/xcfx84=A+BN+CN2.xe2x80x83xe2x80x83Eq.(2)
With increase of the thickness d of the active layer 14, there occurs an increase of carrier density N for a given injection current density, and thus, there occurs a decrease of carrier lifetime xcfx84. Such a decrease of the carrier lifetime xcfx84 contributes to the increase of the saturated optical output power Ps. In Eq.(1), it should be noted that the differential gain gxe2x80x2 decreases with increasing difference (xcexsxe2x88x92xcexp) between the wavelength xcexs of the optical signal and the wavelength xcexp of the gain peak wavelength xcexp. In the case the thickness d of the active layer 14 is decreased, it should be noted that the wavelength xcexp shifts in the direction of short wavelength as a result of the band-filling effect with the increase of carrier density N. As a result, there occurs an increase in the difference xcexsxe2x88x92xcexp and associated decreases of the differential gain gxe2x80x2.
Thus, it is possible to increase the saturated optical output power Ps, and hence the chip-out saturation power, also in the semiconductor optical amplifier such as the one shown in FIG. 2(B) in which a small thickness is used for the active layer 14 as compared with the width, by decreasing the thickness of active layer 14 as represented in FIG. 3.
On the other hand, such a decrease of the thickness d of the active layer 14 causes an increase of the optical confinement factor xcex93te for the Te-polarization mode over the optical confinement factor xcex93tm for the Tm-polarization mode (xcex93te greater than xcex93tm), and there occurs a large polarization dependence in the semiconductor optical amplifier with regard to the Te-polarization mode and with regard to the Tm-polarization mode. It has been known conventionally, that the desired, polarization-independent operation can be realized by introducing appropriate tensile strain into the active layer 14. According to such an approach, it is possible to set the material gain gte for the Te-polarization mode to be smaller than the material gain gtm for the Tm-polarization mode (gte less than gtm), by introducing a tensile strain into the active layer 14. Thereby, it becomes possible to satisfy the condition of polarization-dependence free operation (xcex93texc2x7gte=xcex93tmxc2x7gtm) at least approximately, while using such a flat active layer 14.
As explained previously, FIG. 3 shows, in the vertical axis at the right, the amount of the tensile strain that has to be introduced into the active layer 14 for realizing polarization-independent operation for the semiconductor optical amplifier 10 of Figure, for the case in which the optical confinement layers 13 and 15 are formed to have a thickness of 100 nm and the active layer 14 is formed to have the width of 1.0 xcexcm while changing the thickness of the active layer 14 variously. From FIG. 3, it can be seen that a tensile strain of about 0.2% is necessary in the case the active layer 14 has a thickness of 100 nm. In the case the thickness is 75 nm, on the other hand, it can be seen that the necessary strain is 0.23%. In the case the thickness of the active layer 14 is 50 nm, a tensile strain of 0.25% is necessary. In FIG. 3, it should be noted that the negative strain value represents that the strain is a tensile strain.
FIGS. 4-7 show the gain saturation characteristics of the semiconductor optical amplifier designed according to the foregoing principle, wherein FIG. 4 shows the gain saturation characteristics of the optical semiconductor amplifier 10 of FIG. 1 for the case in which a tensile strain of 0.2% (xe2x88x920.2%) is introduced into the active layer 14 having a thickness d of 100 nm. FIG. 5, on the other hand, shows the gain saturation characteristics of the semiconductor optical amplifier 10 of FIG. 1 for the case in which a tensile strain of 0.23% (strain of xe2x88x920.23%) is introduced to the active layer 14 that has the thickness d of 75 nm. FIG. 6, on the other hand, shows the gain characteristics of the semiconductor optical amplifier 10 of FIG. 1 for the case in which a tensile strain of 0.25% (strain of xe2x88x920.25%) is introduced into the active layer 14 having the thickness d of 50 nm. In FIG. 4-6, it should be noted that the horizontal axis represents the module output optical power while the vertical axis represents the fiber-to-fiber gain of semiconductor optical amplifier 10. Defining the fiber-coupled saturation optical power as the module output optical power that provides a drop of 3 dB for the fiber-to-fiber gain, it can be seen from FIGS. 4-6 that the fiber-coupled saturation optical power takes a value of +12.5 dBm, +14.5 dBm, and +17.0 dBm at the wavelength of 1550 nm respectively for the case in which the active layer 14 has a thickness 100 nm, 75 nm and 50 nm.
As can be seen from FIGS. 4-6, the gain difference between the Te-polarization mode and the Tm-polarization mode is reduced to substantially zero, by introducing the tensile strain into the active layer 14 with an amount explained previously, and a substantially polarization-independent operation is realized for the semiconductor optical amplifier 10.
Thus, in view of the result of FIGS. 4-6, the gain difference between the Te-polarization mode and the Tm-polarization mode is successfully reduced to substantially zero for the optical signals having a wavelength in the vicinity of 1550 nm. However, due to the fact that a large strain is introduced into the active layer 14, the foregoing effect of suppressing the gain difference between the different modes is not effective when the wavelength of the optical signals to be amplified is deviated from the foregoing optimum range. In such a case, therefore, the polarization-independent operation is not obtained.
FIGS. 7-9 shows the gain difference xcex94G between the Te-polarization mode and the Tm-polarization mode of the semiconductor optical amplifier 10 obtained for a wavelength range of 1500 nm-1600 nm, wherein FIG. 7 shows the case of setting the thickness d of the active layer 14 to 100 nm and setting the tensile strain to 0.2%, while FIG. 8 shows the case of setting the thickness d of the active layer 14 to 75 nm and setting the tensile strain to 0.23%. Further, FIG. 9 shows the case of setting the thickness d of the active layer 14 to 50 nm and setting the tensile strain to 0.25%.
FIGS. 7-9 are referred to.
It can be seen that the gain difference xcex94G between the polarization states is very small in the vicinity of the optical wavelength of 1550 nm. On the other hand, when the optical wavelength to be amplified is deviated in the direction of longer wavelength, it can be seen that there appears a substantial gain difference. Moreover, it can be seen that the increase of the gain difference xcex94G between the polarization states is enhanced in the case the thickness d of the active layer 14 is small. For example, in the case the thickness d of the active layer 14 is set to 100 nm, the gain difference xcex94G between the Te-polarization mode and the Tm-polarization mode is about xe2x88x921.1 dB at the wavelength of 1590 nm as for as shown in FIG. 7, while the gain difference xcex94G between the Te-polarization mode and the Tm-polarization mode wavelength reaches a level of 1.5 dB at the wavelength of 1590 nm in the event the thickness d of active layer 14 is reduced to 50 nm as shown in FIG. 9.
In an optical-fiber telecommunication system, the technology of wavelength multiplexing is used for transmitting a large traffic of optical information. Because of this, the spectrum range of the optical signals that are transmitted through an optical-fiber telecommunication system is increasing. Recently, in particular, there is an attempt to extend the transmission band of the optical signals to a longer wavelength side from the conventional 1.55 xcexcm band (C band). Accordingly, the semiconductor optical amplifier for use in such a broadband optical fiber telecommunication system of future has to provide polarization-independent operation over a wide wavelength range. Further, such a semiconductor optical amplifier is required to have a large saturation gain. The conventional semiconductor optical amplifier explained with reference to FIG. 1 cannot meet for such a demand.
Accordingly, it is a general object of the present invention to provide a novel and useful semiconductor optical amplifier wherein the foregong problems are eliminated.
Another and more specific object of the present invention is to provide a polarization-independent optical semiconductor device that operates over a broad optical wavelength band.
Another object of the present invention is to provide a broadband polarization-independent optical semiconductor device that can be fabricated easily by using a bulk active, without the need of narrowing the pattern width of a mesa-stripe structure unrealistically.
Another object of the present invention is to provide a semiconductor optical amplifier, comprising:
a substrate extending from a first end surface to a second end surface;
a first cladding layer formed on said substrate with a first conductivity type;
a plurality of active layers formed on said first cladding layer each having a bandgap smaller than a bandgap of said first cladding layer;
at least one spacer layer interposed between said plurality of active layers and having a bandgap larger than said bandgap of said active layers;
a second cladding layer formed on said substrate so as to cover said plurality of active layers and said at least one spacer layer;
a first electrode injecting carriers to each of said plurality of active layers through said first cladding layer; and
a second electrode injecting carriers to each of said plurality of active layers through said second cladding layer;
each of said plurality of active layers accumulates a tensile strain therein.
Another object of the present invention is to provide a wavelength-multiplexed optical telecommunication system comprising:
a plurality of optical sources having respective, mutually different wavelengths;
a first optical coupler coupling said plurality of optical sources to a single optical fiber;
a semiconductor optical amplifier provided in said optical fiber;
a second optical coupler dividing an optical signal amplified by said semiconductor optical amplifier to a plurality of output optical fibers; and
an optical detector coupled optically to each of said output optical fibers,
said semiconductor optical amplifier comprising:
a substrate extending from a first end surface to a second end surface;
a first cladding layer formed on said substrate with a first conductivity type;
a plurality of active layers formed on said first cladding layer each having a bandgap smaller than a bandgap of said first cladding layer;
at least one spacer layer interposed between said plurality of active layers and having a bandgap larger than said bandgap of said active layers;
a second cladding layer formed on said substrate so as to cover said plurality of active layers and said at least one spacer layer;
a first electrode injecting carriers to each of said plurality of active layers through said first cladding layer; and
a second electrode injecting carriers to each of said plurality of active layers through said second cladding layer;
each of said plurality of active layers accumulates a tensile strain therein,
an input end of said active layer being coupled optically to a first part of said single optical fiber,
an output end of said active layer being coupled optically to a second part of said single optical fiber.
According to the semiconductor optical amplifier of the present invention, the problem of shift of the operational wavelength band of the semiconductor optical amplifier in a short wavelength direction associated with the quantum effect is successfully avoided by using a bulk crystal for the active layers, and an optical gain is obtained in the long wavelength band including the 1.55 xcexcm band. By introducing tensile strain simultaneously, the desired polarization-independent operation is achieved. Further, by interposing the spacer layer between plural active layers, and by optimizing the thickness of the spacer layer, it becomes possible to set the ratio of the optical confinement factors between the Te-polarization mode of and the Tm-polarization mode to approximately 1, while maintaining a large saturation optical output power. In the present invention, it is possible to reduce magnitude of the tensile strain introduced into the active layer for realizing polarization dependent operation of the semiconductor optical amplifier 10.
In the semiconductor optical amplifier of the present invention, it is possible to form an active structure on the surface of the substrate by the plural active layers and the one or more spacer layers and to sandwich the active structure thus formed by a pair of optical confinement layers having a bandgap larger than the bandgap of the active layer. It is preferable that the spacer layer has a thickness of 100 nm or larger, while the spacer layer is preferable to have a thickness of 200 nm or smaller. Further, it is preferable that each of the plural active layers has a thickness exceeding 30 nm, while it is also preferable that each of the active layers has a thickness of 100 nm or less. Particularly, each of the plural active layers is desirable to have a thickness of about 40 nm. The plural active layers may accumulate therein a tensile strain of 0.18% or less. Further, each of the plural active layers is desirable to have a shape in which the width thereof decreases toward the incident end surface and also toward the exit end surface. Alternatively, each of the plural active layers may have a thickness that decreases toward the incident end surface and also toward the exit end surface. In the semiconductor optical amplifier, it is preferable that the plural active layers forms a stripe structure extending form the incident end surface to the exit end surface. Thereby, it is preferable that the stripe structure intersects obliquely with any of the incident end surface and the exit end surface. Further, it is preferable to provide an antireflection coating on the incident end surface and also on the exit end surface.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.